Particle filter devices

ABSTRACT

A device for reducing pollution of an internal combustion engine, including a collection of monolithic elements of honeycomb type connected by a jointing compound, each element incorporating a set of adjacent cells of mutually parallel axes separated by porous walls, which cells are plugged by plugs at one or other of their ends to delimit inlet chambers opening onto a gas intake face and outlet chambers opening onto a gas discharge face such that gas that is to be filtered passes through the porous walls, the collection being inserted in a metal casing by a compacted fibrous mat. A jointing compound has a three-point flexural modulus of rupture of between 0.5 and 6 MPa, the jointing compound has a dynamic Young&#39;s modulus less than or equal to 17 GPa, the mat has a mean density in the compacted state of between 0.30 and 0.54, and the mean thickness of the mat in the compacted state is between 2 and 8 mm.

The present invention relates to the field of devices for filtering theparticles from an internal combustion engine, possibly comprising acatalytic component, particularly installed in an exhaust line of adiesel engine to remove the soot produced by the burning of the fuel.

Diesel engines are known to produce a large amount of soot. This is theresult of phenomena of pyrolysis of hydrocarbon in the absence of oxygenactually within the combustion flame, and from insufficient temperaturewithin the combustion chamber for all of the soot particles thusproduced to be burned off. This soot, when emitted from the vehicle,acts as seeds on which unburnt hydrocarbons condense, thus forming solidparticles that can be inhaled and the small size of which allows them toprogress as far as the pulmonary alveoli.

In order to limit the emission of soot out of the vehicle and to meetever tighter environmental emissions standards, it is known practice forfiltration devices, possibly associated with catalytic devices, to bepositioned in the exhaust line, these catalytic devices having thepurpose of converting the pollutant gaseous emissions into inert gases.Featuring strongly among the pollutant gaseous emissions are the unburnthydrocarbons together with oxides of nitrogen (NOx) or carbon monoxide(CO).

Soot filtration devices comprise “particulate filters” which generallyconsist of a filtering support made of porous ceramic. This supportgenerally has a honeycomb structure, one of the faces of said structureadmitting the exhaust gases that are to be filtered and the other facedischarging the filtered exhaust gases. Between these faces, thefiltering structure has a set of longitudinal and mutually parallelcells separated by porous walls, said cells being plugged at one oftheir ends to force the exhaust gases to pass through said porous walls.To ensure that the entity is correctly sealed, the peripheral part ofthe structure is surrounded with a cement known as a coating cement. Thefilter is also placed in a can, this often being known as “canning”consisting of a fibrous mat and of a metal casing. In order to affordbetter resistance to thermal shock, the filters are sometimes made up ofa collection of monolithic and parallelepipedal elements of honeycombstructure, said elements being assembled using a material known as a“jointing compound”.

The ceramics most often used are cordierite (Mg₂Al₄Si₂O₁₈) or siliconcarbide (SiC), the latter being preferred for its thermal conductivityand corrosion-resistance properties. Silicon carbide filters arepreferably obtained by sintering, for example SiC filters connected bysintered silicon or those obtained by recrystallization (R—SiC).Examples of filters are described for example in patent applications EP816 065, EP 1 142 619, EP 1 455 923 or alternatively WO 2004/065088 towhich reference may be made for greater details regarding theirstructure or method of synthesis.

During engine operation, the particulate filter becomes laden with sootparticles which are deposited on the porous walls. The problem of theminimum temperature needed to burn off the soot arises here just as itdoes in the combustion chamber. Because the soot is held in the filter,the combustion dynamics may be slower than in the combustion chamber,making it possible to lower the temperature at which the soot burns offto about 600° C. However, this reduction is not enough to ensure thatthe soot is burnt off within the filter throughout the entire engineoperating range. It is therefore necessary, after a filtration cycle, toprovide a regeneration cycle during which the soot is burnt.

The particulate filter therefore operates in the following modes:

-   -   filtration and quasi-simultaneous combustion of soot when the        temperature of the exhaust gases so permits,    -   retention and accumulation of soot particles in the filter when        the temperature of the exhaust gases is too low,    -   regeneration of the filter before the pressure drops caused by        the build-up of soot become unacceptable.

The progressive plugging of the filter during the soot retention phasedoes in fact lead to an increase in pressure drop resulting in anincrease in engine fuel consumption, or even a raised back pressurewhich may damage the combustion system.

The regeneration step is performed by raising the temperature of theexhaust gases using post injection, which consists in a late injectionin the engine cycle of fuel which will burn in the exhaust line.

During regeneration, and because of the exothermic combustion of soot,the filter experiences high temperatures, and what is more, itexperiences temperatures that are not uniform within the material,because the particles of soot prefer to deposit themselves in thecentral part of the filter and in the downstream part thereof. Thefilter is therefore subjected to intense radial and tangentialthermomechanical stresses liable to give rise to microcracks within thematerial leading to a partial or even complete loss of its filtrationcapability.

In general, filter improvements involve obtaining the best possiblecompromise between the following properties for equivalent enginespeeds. In particular, it is an object of the invention to provide afiltration device formed of an assembly of monolithic elements whichsimultaneously exhibits:

-   -   a low pressure drop caused by a structure that is filtering in        operation, that is to say typically when this structure is in        the exhaust line of an internal combustion engine both when said        structure is free of soot and when it laden with particles,    -   a high soot storage volume so as to reduce the frequency of        regeneration periods,    -   a filter mass best suited to ensure sufficient thermal mass so        that the maximum regeneration temperature and the gradients        experienced by the filter can be minimized,    -   a high thermomechanical strength, that is to say one that gives        the filtration device an extended life.

The performance of the filtering devices comprising a filter inserted ina metal casing by means of a fibrous mat is, for its part, characterizedby the following properties:

-   -   the mechanical integrity of the device: the individual        monolithic elements of the filter, the fibrous mat and the metal        casing have to remain joined together after the device has been        subjected to vibrations, particularly to vibrations        representative of those experienced by such a device in an        exhaust line of an engine. Insufficient mechanical integrity may        manifest itself in disconnection of the fibrous mat and of the        filter or of the mat with respect to the metal casing, or        alternatively in disconnection of one or more monolithic        elements of an assembled filter.    -   sealing against the hot gases that are to be filtered: the        passage of soot through the mat, between the mat and the filter        or between the mat and the metal casing need to be avoided.

It would seem important to be able to obtain a device that is able tosolve all of the aforementioned problems, particularly a device that hasimproved thermomechanical strength and improved mechanical integrity.

The inventors have discovered the key parameters that are necessary andsufficient for obtaining such a device.

In its most general form, the subject of the present invention is adevice for reducing the pollution of an internal combustion engine,comprising a collection of monolithic elements of the honeycomb typeconnected by a jointing compound, each element incorporating a set ofadjacent cells of mutually parallel axes separated by porous walls,which cells are plugged by plugs at one or other of their ends todelimit inlet chambers opening onto a gas intake face and outletchambers opening onto a gas discharge face such that the gas that is tobe filtered passes through the porous walls, said collection beinginserted in a metal casing by means of a compacted fibrous mat. Thedevice according to the invention is characterized in that:

-   -   the jointing compound has a three-point flexural modulus of        rupture of between 0.5 and 6 MPa, preferably between 1 and 5        MPa, particularly between 2 and 4 MPa,    -   the jointing compound has a dynamic Young's modulus less than or        equal to 17 GPa, preferably less than or equal to 10 GPa,    -   the mat has a mean density in the compacted state of between        0.30 and 0.54, preferably less than or equal to 0.50,    -   the mean thickness of the mat in the compacted state is between        2 and 8 mm.

It is in fact thanks to a careful combination of these variousparameters that the filtration device according to the invention is ableto solve the various abovementioned problems.

The porous walls are preferably made of a ceramic material, typicallymade of cordierite (Mg₂Al₄Si₂O₁₈), of aluminum titanate, or based onsilicon carbide (SiC), the latter being preferred for its thermalconductivity and corrosion resistance properties. What is meant withinthe meaning of the present description by “material based on SiC” isthat said material contains at least 30 wt % of SiC, preferably at least70 wt % of SiC and as an extreme preference at least 98 wt % of SiC.

The material of which the walls are made preferably has an open porosityof between 35 and 65%, and more preferably still of between 40% and 60%.Particularly in an application to a particulate filter, too low aporosity leads to too high a pressure drop. Too high a porosity bycontrast leads to too low a mechanical strength. The median diameterd50, by volume, of the pores constituting the porosity of the materialpreferably ranges between 5 and 25 microns, particularly between 10 and30 microns. In general, in the intended applications, it is generallyaccepted that too low a pore diameter leads to too high a pressure drop,whereas too high a median pore diameter leads to poor filtrationefficiency.

In general, the cross section of a monolithic element that makes up theassembled structure is square, the width of the element ranging between30 mm and 50 mm. Advantageously, the thickness of the walls rangesbetween 200 and 500 μm. The number of cells in the filtering elementspreferably ranges between 7.75 and 62 per cm², said cells having a crosssection of about 0.5 to 9 mm². The cells may have various shapes. Theymay have identical or different shapes and sizes, particularly being ofsquare, hexagonal, octagonal or triangular shape. The cells may, forexample, all be square and of the same size. They may also, for example,alternate between square and hexagonal and square and octagonal shapes.The cells may also have more complex shapes associated with acorrugation of the walls, as described for example in application WO05/016491.

The filters are preferably such that the total volume of the inletchambers opening onto the gas inlet face is greater than the totalvolume of the outlet chambers opening onto the gas discharge face. Forexample, the inlet cells may be more numerous than the outlet cells(particularly if the inlet cells and outlet cells all have the samecross-sectional area) and/or the inlet cells may have a largercross-sectional area than the outlet cells (particularly if the numberof inlet cells is equal to the number of outlet cells). What is meant byinlet cells and outlet cells respectively, are the cells open onto theinlet face and onto the discharge face for the gases, respectively. Suchfilters, known as asymmetric filters, have the advantage that they areable to store a greater quantity of soot, making it possible to lengthenthe time between two successive regenerations and to reduce the increasein pressure drop as the filter becomes laden with soot. Implementationof the invention has proven to be particularly advantageous in the caseof such filters because the inventors have been able to demonstrate thatsuch filters were more liable to be affected by higher thermomechanicalstresses than standard filters.

The mean thickness of the jointing compound is preferably between 0.5and 4 mm, particularly at least 1 mm. For small thicknesses, themechanical integrity of the filter is poor and the spread on flatness ofthe monolithic elements may then generate local thermomechanicalstresses and reduce the relaxation of stresses by the jointing compound.If the thickness is too high, the pressure drop of the filter becomestoo great, especially when there are a great many monolithic elements,that is to say when the number of joints across the cross section of thefilter perpendicular to the axis of the filter is high.

The jointing compound is understood here to mean a moldable compositionformed of a wet or dry particulate and/or fibrous mix, able to set solidand to have sufficient mechanical strength at ambient temperature orafter drying and/or heat treatment the temperature of which will notexceed the softening or collapse temperature which defines therefractoriness of the material or materials of which the monolithicelements are made.

What is meant by “moldable” is a composition capable of plasticdeformation needed to spread over the face of the joint of themonolithic elements and which exhibits sufficient adhesion with respectto these elements so that it can hold them together or allow the filterto be handled in its assembled state immediately after the jointingoperation or, where necessary, after a heat treatment or chemicaltreatment or some other treatment such as ultraviolet irradiation.

The jointing compound preferably contains particles and/or fibers ofceramic or of refractory material, chosen from non-oxides, such as SiC,aluminum and/or silicon nitride, aluminum oxynitride, or from amongoxides, particularly including Al₂O₃, SiO₂, Cr₂O₃, MgO, ZrO₂, or anymixture thereof.

For preference, the composition contains at least 20% SiC. To encourageit to harden, the jointing compound preferably contains a thermosettingresin, in a quantity of at least 0.05 wt % and at most 5 wt % withrespect to the mineral filler. A catalytic hardener intended toaccelerate the setting of the resin, preferably also in the form of apowder, may be added to the mixture. The jointing compound may containclay to encourage plasticity and its moldable nature. The jointingcompound may also contain inorganic fibers and organic and/or inorganicbinders. What is meant by an organic binder is, in particular, temporarybinders such as derivatives of cellulose or of lignin, such ascarboxymethylcelluloses, dextrin or alternatively polyvinyl alcohols.What is meant by inorganic binders is, in particular, chemical settingagents such as phosphoric acid, aluminum monophosphate or sols based onsilica and/or on alumina and/or on zirconia or possibly sinter promoterssuch as titanium dioxide or magnesium hydroxide, or even shaping agentssuch as calcium stearate or magnesium stearate. The jointing compound ispreferably a ceramic and/or refractory cement.

For preference, the filtering monolithic elements are based on SiC andare assembled by a jointing compound the thermal conductivity of whichis greater than or equal to 0.1 W/m·K for all temperatures between 20and 800° C. A high thermal conductivity of the jointing compoundadvantageously makes it possible to even out the heat transfers withinthe filter whereas a low thermal conductivity, particularly one of below0.1 W/m·K (typically measured at a temperature of 600° C.) contributesto increasing the temperature gradients and the thermomechanicalstresses in the joint and within the filter.

The monolithic elements are preferably assembled by partial bonding,inasmuch as the space between the monolithic elements may be notcompletely filled by the jointing compound, so as to relax thethermomechanical stresses in the filter, as described for example inapplications EP 1 726 800 or FR 2 833 857. Jointing compoundconfigurations like those described in applications WO 2005/084782 or WO2004/090294, which involve regions of low or zero adhesion between thejointing compound and the filtering element and regions of strongadhesion between the jointing compound and the filtering element arealso conceivable.

The assembled filter preferably has a coating cement secured to theassembled filter, particularly having the same mineral composition asthe jointing compound, so as to reduce thermomechanical stresses.

The pollution-reducing device may further comprise a catalytic coatingto treat pollutant gases of the CO or HC and/or NOx type.

The fibrous mat is preferably formed of inorganic fibers so as to affordthe thermal insulation properties required for this application. Theinorganic fibers are preferably ceramic fibers, such as fibers ofalumina, of mullite, of zirconia, of titanium oxide, of silica, ofsilicon carbide or nitride, or alternatively glass fibers, for exampleglass R fibers. These fibers may be obtained by fiber drawing from abath of molten oxides, or from a solution of organo-metallic precursors(the sol-gel method). The fibrous mat is preferably non-intumescent. Itis advantageously in the form of a needled felt.

The density of the mat in the compacted state is dependent in particularon the mass per unit volume of the material of which this mat is madeprior to compaction and on the thickness of the mat after compaction.Mats capable of exhibiting the required densities in the compacted stateare, for example, marketed by the company Saffil Ltd, under thereferences 1600, 1250 or 2400 or alternatively by the company IbidenCo., Ltd, under the references N4-1515 or N4-1253.

Particularly in the case of filters of non-circular cross section, thedensity in the compacted state and/or the thickness of the mat isadvantageously non-uniform, in as far as it may vary according to theregion of the space formed between the filter and the metal casing.During regeneration, this type of filter is in fact liable to exhibit anon-uniformity of temperature at its periphery. The difference betweenthe temperature of certain regions of the periphery of the filter andthe temperature at the center of the filter may thus be 20% or moregreater than the mean difference between the peripheral temperature andthe temperature at the center of the filter, and this non-uniformity oftemperature is likely to give rise to high very localized stressconcentrations in these regions. In order to achieve a more favorabledistribution of thermomechanical stresses, the density of the mat istherefore preferably lower than the mean density and/or the thickness ofthe mat is preferably higher than the mean thickness in contact with theregions at which the thermomechanical stresses may become concentratedduring the regeneration phases. In order best to optimize the overallthermomechanical strength of the filtration device according to theinvention, the thickness of the mat in the compacted state at theperipheral regions of the filter which are subjected to the highestthermomechanical stresses is preferably at least 20%, particularly atleast 50%, and even at least 100% higher than the thickness of the matat the peripheral regions subjected to the lowest thermomechanicalstresses. Alternatively or in combination, the density of the mat in thecompacted state at the peripheral regions of the filter subjected to thelowest stresses is preferably at least 20%, particularly at least 50%and even at least 100% higher than the density at the regions subjectedto the highest stresses. The method of the “shrinking” type (where themetal casing is shrunk around the mat) allows the density and/or thethickness of the mat to be modified, creating regions of lower densityand/or greater thickness in these regions that are liable to be mosthighly affected by this stress concentration. In the case of anellipsoidal or substantially ellipsoidal filter, it is particularlypreferable for the density of the mat to be lower and/or for thethickness of the mat to be higher at the ends of the minor and of themajor axes of the ellipse, these ends being the most highlythermomechanically stressed during regeneration.

When the density in the compacted state is non-uniform, the measurementtaken corresponds to a mean value.

The mean thickness of the fibrous mat, in the compacted state, isdetermined on the filter placed in its metal casing, by calculating themean of 4 thickness measurements taken in a plane perpendicular to theaxis of the filter on 4 segments of two mutually perpendicular straightlines passing through the geometric center of the filter.

The density of the mat in the compacted state may be measured asfollows: the filter surrounded by its mat is taken out of its metalcasing then unrolled so that its surface area can be measured, andweighed so as to measure its relative density in g/cm². The density inthe compacted state is obtained by dividing the previously determinedrelative density by the mean thickness of the mat in cm.

Insertion into the metal casing can be done using various methods knownto those skilled in the art. Mention may be made in particular of themethods known as the “tourniquet” method, the “shrinking” method, the“clamshell” method or the “stuffing” method.

The modulus of rupture of the jointing compound is measured at ambienttemperature on a test specimen measuring 150×25×25 mm³. The setup for3-point flexural testing in accordance with the standard NF B41-104 isperformed with a distance of 120 mm between the two lower supports andthe rate of descent of the loading plunger is equal to 0.5 mm/min. Thevalue is a mean calculated from three successive measurements.

The dynamic Young's modulus is measured, in accordance with standardASTM C1259-01, on test specimens of the same dimensions as those usedpreviously, using test apparatus marketed under the referenceGrindosonic MK5 by the company J.W. Lemmens. The dynamic Young's modulusis determined by measuring the natural frequency of flexural vibrationat ambient temperature of a test specimen of the jointing compound inso-called “dynamic” mode. The test specimen is placed on two supports ofthe rubbery type, so as not to interact with the vibration mode of thetest specimen being tested. The supports are positioned symmetricallywith respect to the center mid-way along the test specimen. The distancebetween supports is 100 mm. The test specimen is excited by a mechanicalimpulse as close as possible to its center on its upper face theopposite face to the face resting on the supports, for example using astick or a pencil or a small hammer supplied with the apparatus, becausethe excitation energy needed is small. This excitation leads tovibrations within the material of the test specimen. A piezoelectricdetector positioned in contact with the test specimen then records thesevibrations and converts them into an electrical signal from which thenatural frequency of vibration is displayed.

The dynamic Young's modulus E is then calculated (in GPa) as a functionof the mass m (in g) of the test specimen and of the flexural resonantfrequency f (in Hz) using the following formula:

E=9.1584×10⁻⁹ ×m×f ²

All the measurements (density, thickness, modulus of rupture and Young'smodulus) are taken at ambient temperature.

To measure the moduli, the test specimen of jointing compound isprepared by molding the composition, and it then undergoes the sametreatment (for example a heat treatment) as is undergone by the jointingcompound when used to assemble the monolithic elements with one another,finally being dried at 110° C. before being cooled to ambienttemperature.

The invention and its advantages will be better understood from readingthe examples which follow. Of course, these examples must not beconsidered, in any of the aspects described, as limiting the presentinvention.

FIGS. 1 and 2 schematically depict non-circular filters 1 formed of aplurality of elements 2. The hatched regions 3 represent the peripheralregions in which the difference in temperature by comparison with thetemperature at the center of the filter is likely, during regeneration,to be 20% or more greater than the mean difference between theperipheral temperature and the temperature at the center of the filter.This non-uniformity of temperature is likely to give rise to high verylocalized stress concentrations in these regions. It is thereforeadvantageous for the density of the mat to be lower near this regionwhere the thickness of the mat is greater.

EXEMPLARY EMBODIMENTS

In the examples which follow, a series of filtering devices according tothe invention and which illustrate its advantages over another series ofdevices given for comparison purposes and which do not meet the criteriaof the invention, were created.

All the monolithic filtering elements were created using the followingmethod.

Using a mixer, powders of silicon carbide, a pore-generating agent ofthe polyethylene type and an organic binder of methylcellulose type werefirst of all mixed. Water was added and mixing was continued until auniform paste was obtained with a plasticity that allowed extrusionthrough a square section honeycomb monolithic structure die thedimensional characteristics of which are given in table 1.

The raw elements obtained were then dried using microwaves for longenough to bring the chemically unbound water content down to under 1 wt%.

The cells on each face of the blocks were then alternately plugged usingwell known techniques, for example described in application WO2004/065088.

The elements were then baked at an increase in temperature of 20° C./huntil a temperature of the order of 2200° C. was obtained, thistemperature then being maintained for 2 hours.

This finally yielded a series of monolithic filtering elements made ofsilicon carbide, the microstructural characteristics of which weresubstantially identical.

TABLE 1 Cell geometry Square Cell density 180 cpsi (cells per squareinch, 1 inch = 2.54 cm) Wall thickness 350 μm Length 15.2 cm Width 3.6cm Porosity About 47% Median pore diameter About 15 μm

According to the teaching of patent application EP 816 065, 16 filteringmonolithic elements were then assembled with one another by bondingusing a jointing compound of ceramic nature and were then machined toform filters of a suitable diameter. The thickness of the jointingcompound was 1 mm.

In the case of comparative examples C1 to C3 and of the exampleaccording to the invention 1, the jointing compound was prepared bymixing the compound J1:

-   -   81 wt % of a powder of SiC with a particle size of between 10        and 200 μm,    -   4 wt % of a powder of calcined alumina the median diameter of        which was about 5 microns, marketed by the company Almatis,    -   8 wt % of a powder of reactive alumina the median diameter of        which was about 3 microns, marketed by the company Almatis,    -   6% of silica fume of the Elkem 971 type,    -   0.8 wt % of a temporary and plasticizing binder of the cellulose        type,    -   0.2 wt % of a deflocculant of the STPP (sodium tripolyphosphate)        type.

A quantity of water corresponding to about 15% of the weight of thismixture was added in order to obtain a paste of suitable viscosity.

Once the filter had been machined, a coating cement of the same mineralcomposition as used for the jointing compound was applied to thecylindrically shaped filters with a volume of the order of 2.48 liters.The assembled filter was then subjected to a heat treatment in air at750° C. with the maximum temperature sustained for 2 h. In the case ofcomparative example C3, the heat treatment was performed at atemperature of 950° C. instead of 750° C., which had the effect ofincreasing the modulus of rupture and Young's modulus of the jointingcompound.

In the case of comparative example C4 and of the examples according tothe invention 2 and 3, the jointing compound was prepared by mixing thefollowing compound J2:

-   -   67 wt % of a powder of SiC with a particle size ranging between        10 and 200 μm,    -   3 wt % of a powder of reactive alumina marketed by the company        Almatis, the median diameter of which was about 3 microns,    -   24% of hollow spheres marketed by Enviro-spheres under the name        “e-spheres” which have a typical chemical composition containing        60% SiO₂ and 40% Al₂O₃ and a median diameter of the order of 100        μm,    -   6% of silica fume of the Elkem 971 type,    -   0.8 wt % of a temporary and plasticizing binder of the cellulose        type,    -   0.2 wt % of a deflocculant of the STPP (sodium tripolyphosphate)        type.

A quantity of water corresponding to about 15% of the weight of thismixture was added in order to obtain a paste of suitable viscosity.

Once the filter had been machined, a coating cement of the same mineralcomposition as used for the jointing compound was applied to thecylindrically shaped filters with a volume of the order of 2.48 liters.The assembled filter was then subjected to a heat treatment in air at950° C. with the maximum temperature sustained for 2 h.

The filters were then coated with various fibrous mats then inserted intheir metal casing in accordance with the teaching associated with FIG.5 of patent application EP 1 382 374 (the so-called “tourniquet” method)in order to obtain densities in the compacted state and mat thicknessesin the compacted state as collated in table 2.

The metal casing was made up of two parts formed of 13% chromiumrefractory stainless steel sheets 1.5 mm thick.

The devices thus obtained were subjected to the followingcharacterization tests.

A) Thermomechanical Strength Test

The devices were mounted on an exhaust line of a 2.0 l direct-injectiondiesel engine run at full power (4000 rpm) for 30 minutes then removedand weighed in order to determine their initial mass. The devices werethen re-fitted on the engine test bed with an engine speed of 3000 rpmand a torque of 50 Nm for various lengths of time in order to obtain asoot loading of 8 g/liter (in terms of filter volume). The devices thusladen with soot were re-fitted on the line to undergo severeregeneration defined as follows: after stabilizing at an engine speed of1700 rpm for a torque of 95 Nm for 2 minutes, post-injection wasperformed with a 70° phase angle for a post-injection delivery of 18mm³/shot. Once combustion of the soot was initiated, more specificallyonce the pressure drop had decreased for at least 4 seconds, the enginespeed was lowered to 1050 rpm for a torque of 40 Nm for 5 minutes inorder to accelerate the combustion of the soot. The devices were thensubjected to an engine speed of 4000 rpm for 30 minutes in order toeliminate the remaining soot.

The regenerated filters were inspected after slicing to reveal thepresence of any cracks there might be that were visible to the nakedeye. The thermomechanical strength of the filter was assessed in thelight of the number of cracks, a low number of cracks meaning athermomechanical strength acceptable for use as a particulate filter.

As collated in table 2, the following scores were assigned to each ofthe filters:

-   -   +++: very numerous cracks present,    -   ++: numerous cracks present,    -   +: a few cracks present,    -   −: no cracks or very rare cracks.

Because the severe regeneration was characterized by particularlyextreme conditions, the presence of a few cracks (the score “+”) isacceptable. Scores “++” and “+++” by contrast are representative of poorthermo-mechanical strength.

B) Method of Assessing Integrity

The device comprising the filter with its metal casing and its fibrousmat was placed on an electrodynamic test rig equipped withaccelerometers positioned at various points. A first accelerometer wasplaced in contact with the filter at the center of one of the planarfaces thereof, a second accelerometer being positioned on the metalcasing of the canning. These two accelerometers, which were at leasttwo-axes accelerometers, are able to measure vibration in the directionof the axis of the filter and radial vibrations and any decouplingbetween the filter and its canning and to monitor the stability of theattachment of the canned filter to the electrodynamic test rig. Thefilter was subjected to a cycle of vibration at a frequency of 185 Hzcomprising successive 15-minute levels each corresponding to a givenacceleration. The first level corresponded to an acceleration of 5 G,the second to an acceleration of 10 G, the acceleration then beingincreased in steps of 10 G for each successive level. This vibrationtest can be carried out on an electrodynamic test rig marketed by thecompany LDS Test and Measurement LLC, with a capacity of 35 kN andequipped with a hydraulic ram with a maximum force of 10 kN operating inthe frequency range 0-500 Hz and a 200 bar hydraulic system with a flowrate of 21 l/min.

The device was then subjected to a filtration efficiency test. Thefiltration efficiency of the filtering device after vibration test wasdetermined by measuring the amount of smoke emitted at the outlet of thefilter by comparison with the quantity at the inlet. To do this, a smokemeter was positioned upstream and downstream of the filtering device,the latter being positioned on an exhaust line of a diesel engine. Thesmoke meter made it possible to determine the amount of particles ofsoot emitted by measuring the blackening due to the smoke. During themeasurement, the engine was preferably set at its operating pointcorresponding to its maximum power. If the filtering device hassufficient integrity characteristics, the filtration efficiency indexneeds to remain higher than 85%.

Table 2 below sets out, for comparative examples C1 to C4 and examplesaccording to the invention 1 to 3, the following properties:

-   -   the nature of the jointing compound (J1 or J2, using the coding        given hereinabove),    -   the temperature (in ° C.) and the length (in hours) of the heat        treatment after assembly,    -   the modulus of rupture, termed “MOR”, measured using the method        described hereinabove, and expressed in MPa,    -   the dynamic Young's modulus, termed “MOE”, measured according to        the method described hereinabove, and expressed in GPa,    -   the density of the mat in the compacted state, measured        according to the method described hereinabove,    -   the mean thickness of the mat in the compacted state, measured        according to the method described hereinabove, and expressed in        mm,    -   the results of the thermomechanical strength test,    -   the results of the test of integrity after vibration: the sign        “X” means that the integrity of the filter was not affected by        the test, and the symbol “O” means the opposite,    -   the efficiency of the filter after the integrity test, expressed        in %.

TABLE 2 C1 C2 C3 C4 1 2 3 Jointing J1 J1 J1 J2 J1 J2 J2 compound Heattreatment 750° C. 750° C. 950° C. 950° C. 750° C. 950° C. 950° C. afterassembly 2 h 2 h 2 h 2 h 2 h 2 h 2 h MOR (MPa) 3 3 8 3 3 3 3 MOE (GPa)15 15 20 6 15 6 6 Mat density 0.56 0.35 0.35 0.2 0.35 0.52 0.35 Matthickness 6 1.5 6 3 6 3 6 (mm) Thermomechanical +++ ++ +++ + + + −strength Vibration test X X X ◯ X X X Post-vibration >85% >85% >85%<65% >85% >85% >85% filtration efficiency

The various examples and comparative examples illustrate the fact thatthe four essential characteristics of the invention have to be metsimultaneously, and therefore in combination, in order to solve all ofthe aforementioned technical problems. Choosing too high a density forthe mat in the compacted state (example C1) leads to too low athermomechanical strength as illustrated by the presence of verynumerous cracks after severe regeneration, in spite of the choice of ajointing compound with a low modulus of rupture and Young's modulus. Thesame is true when the thickness of the mat in the compacted state is toosmall (example C2) in spite of the choice of a suitable density.Conversely, too low a density (example C4) is detrimental to obtaininggood integrity: the filter obtained is unable to withstand highvibrations, causing the various elements of the device to becomedetached and leading to a significant drop in filtration efficiency.

Finally, choosing a suitable mat thickness and a suitable mat densitydoes not make the filter acceptable in terms of thermomechanicalstrength if on the other hand, the Young's modulus and modulus ofrupture of the jointing compound are too high (example C3). Asillustrated by the examples according to the invention 1 to 3, it isindeed the combination, firstly of a mat of suitable density andthickness and secondly of a jointing compound that has suitable Young'smodulus and suitable modulus of rupture that makes it possible to obtaina truly high-performance filtration device.

The foregoing description illustrates a few possible embodiments of theinvention. Of course, this description is not however limiting and theperson skilled in the art will be able to devise other variants of theinvention without thereby departing from the scope thereof.

1-11. (canceled)
 12. A device for reducing pollution of an internalcombustion engine, comprising: a collection of monolithic elements ofhoneycomb type connected by a jointing compound, each elementincorporating a set of adjacent cells of mutually parallel axesseparated by porous walls, which cells are plugged by plugs at one orother of their ends to delimit inlet chambers opening onto a gas intakeface and outlet chambers opening onto a gas discharge face such that gasthat is to be filtered passes through the porous walls, the collectionbeing inserted in a metal casing by a compacted fibrous mat, wherein thejointing compound has a three-point flexural modulus of rupture ofbetween 0.5 and 6 MPa, wherein the jointing compound has a dynamicYoung's modulus less than or equal to 17 GPa, wherein the mat has a meandensity in a compacted state of between 0.30 and 0.54, wherein the meanthickness of the mat in the compacted state is between 2 and 8 mm. 13.The device as claimed in claim 12, wherein the modulus of rupture isbetween 1 and 5 MPa, or is between 2 and 4 MPa.
 14. The device asclaimed in claim 12, wherein the dynamic Young's modulus is less than orequal to 10 GPa.
 15. The device as claimed in claim 12, wherein the meandensity in the compacted state is less than or equal to 0.50.
 16. Thedevice as claimed in claim 12, wherein the porous walls are made of aceramic material based on silicon carbide (SiC).
 17. The device asclaimed in claim 16, wherein thermal conductivity of the jointingcompound is greater than or equal to 0.1 W/m·K between 20 and 800° C.18. The device as claimed in claim 12, wherein the mean thickness of thejointing compound is between 0.5 and 4 mm.
 19. The device as claimed inclaim 12, wherein the monolithic elements are assembled by partialbonding.
 20. The device as claimed in claim 12, wherein the density inthe compacted state and/or the thickness of the mat is non-uniform, suchthat the density of the mat is lower than the mean density and/or thethickness of the mat is higher than the mean thickness in contact withregions where thermomechanical stresses may become concentrated duringregeneration phases.
 21. The device as claimed in claim 20, wherein thethickness of the mat in the compacted state at peripheral regions of thefilter subjected to a highest thermomechanical stresses is at least 20%,or is at least 50%, or at least 100% higher than the thickness of themat at peripheral regions subjected to lowest thermomechanical stresses,and/or the density of the mat in the compacted state at the peripheralregions of the filter subjected to the lowest stresses is at least 20%,or is at least 50%, or at least 100% higher than the density in theregions subjected to the highest stresses.
 22. The device as claimed inclaim 12, further comprising a catalytic coating to treat pollutinggases of CO or HC and/or NOx type.